evaluation of the impacts of marine salts and asian dust on the forested yakushima island ecosystem a world natural heritage site in japan

This article is published with open access at Springerlink.com Abstract To elucidate the influence of airborne mate-rials on the ecosystem of Japan’s Yakushima Island, we determined the

Evaluation of the Impacts of Marine Salts and Asian Dust

on the Forested Yakushima Island Ecosystem, a World

Natural Heritage Site in Japan

Takanori Nakano&Yoriko Yokoo&

Masao Okumura&Seo-Ryong Jean&

Kenichi Satake

Received: 1 May 2012 / Accepted: 15 August 2012 / Published online: 12 September 2012

# The Author(s) 2012 This article is published with open access at Springerlink.com

Abstract To elucidate the influence of airborne

mate-rials on the ecosystem of Japan’s Yakushima Island, we

determined the elemental compositions and Sr and Nd

isotope ratios in streamwater, soils, vegetation, and

rocks Streamwater had high Na and Cl contents, low

Ca and HCO3 contents, and Na/Cl and Mg/Cl ratios

close to those of seawater, but it had low pH (5.4 to

7.1), a higher Ca/Cl ratio than seawater, and distinct

87

Sr/86Sr ratios that depended on the bedrock type

The proportions of rain-derived cations in streamwater,estimated by assuming that Cl was derived from sea saltaerosols, averaged 81 % for Na, 83 % for Mg, 36 % for

K, 32 % for Ca, and 33 % for Sr The Sr value wascomparable to the 28 % estimated by comparing Srisotope ratios between rain and granite bedrock Thesoils are depleted in Ca, Na, P, and Sr compared withthe parent materials At Yotsuse in the northwesternside, plants and the soil pool have87Sr/86Sr ratios similar

to that of rainwater with a high sea salt component Incontrast, the Sr and Nd isotope ratios of soil minerals inthe A and B horizons approach those of silicate minerals

in northern China’s loess soils The soil Ca and P tion results largely from chemical weathering of plagio-clase and of small amounts of apatite and calcite ingranitic rocks This suggests that Yakushima’s ecosys-tem is affected by large amounts of acidic precipitationwith a high sea salt component, which leaches Ca and itsproxy (Sr) from bedrock into streams, and by Asiandust-derived apatite, which is an important source of P

deple-in base cation-depleted soils

Keywords Yakushima Asian dust Stream water Chemical weathering Sr isotope Nd isotope

Ca depletion

1 IntroductionThe atmosphere of the Japanese archipelago is rich inmarine aerosols from the surrounding ocean and has

DOI 10.1007/s11270-012-1297-z

T Nakano (*)

Research Institute for Humanity and Nature,

457-4 Kamigamo Motoyama, Kita-ku,

been adversely affected by acidic pollutants and dust

minerals transported from the Asian continent

(Hatakeyama et al.2004; Shimizu et al.2004; Inoue

et al 2005; Nakano et al 2006; Seto et al 2007;

Hartmann et al.2008) Monitoring studies over more

than 10 years have shown the acid rain impact on soil

and aquatic ecosystems in the mountainous area of

Japan (e.g., Kurita and Ueda 2006; Nakahara et al

2010) However, few researchers have evaluated the

impacts of atmospheric deposition of

continental-derived materials on Japan’s terrestrial and aquatic

ecosystems The effects of rain and aerosols on

bio-geochemical cycles are so complex that an integrated

approach that considers entire ecological systems as

single interacting units is required to understand these

effects Nutrients and other ions in the soil–vegetation

system and in terrestrial water are ultimately derived

not only from the atmosphere but also from

weather-ing of the soil and the underlyweather-ing bedrock

Accordingly, identification and quantification of

atmosphere- or bedrock-derived materials in plants,

soils, and streamwater are important for assessing the

biogeochemical cycles in terrestrial ecosystems

Rainwater in Japan has87Sr/86Sr ratios that clearly

differ from those of the substrate rocks at depositional

sites, and it contains high quantities of Sr and Ca that

are derived from acid-soluble minerals (mainly

calci-um carbonate) that originated in the desert and loess

areas of northern China (Nakano and Tanaka 1997;

Nakano et al.2006) Sr is a good proxy for Ca (Miller

et al.1993;Ǻberg 1995; Clow et al.1997), which is

essential for plant growth (as are K, P, and Si), and the

87

Sr/86Sr ratios of water and vegetation are affected by

the ratios present in a basin’s bedrock (Graustein

1988; Faure and Mensing 2005) The 87Sr/86Sr ratio

and concentrations of dissolved ions in rainwater show

temporal variation (Nakano and Tanaka1997; Nakano

et al.2006), whereas those of a stream’s base flow are

temporally invariant and can therefore be considered

to represent year-round water characteristics (Rose

and Fullagar 2005) Accordingly, Sr isotopes have

been utilized as powerful tracers for determining the

sources and flows of Ca within soil–vegetation

sys-tems (e.g., Miller et al.1993;Ǻberg1995; Blum et al

2002) and aquatic systems (e.g., Clow et al 1997;

Shand et al.2009) Nd isotopes also have considerable

potential as atmospheric and environmental tracers,

since the soils in northern China are reported to have

143

Nd/144Nd ratios (ε values) that are distinct from

those of many rocks in Japan (Bory et al 2003;Nakano et al.2004) Several Sr and Nd isotope studieshave shown that Asian dust minerals are deposited inthe soils of southwestern Japan (Mizota et al 1992)and Hawaii (Chadwick et al.1999; Kurtz et al.2001),but few studies have used both isotopes as biogeo-chemical tracers in terrestrial systems (Pett-Ridge et

al.2009)

Yakushima Island, in southwestern Japan (Fig.1),became a world natural heritage site in 1993 in recog-nition of its unique and irreplaceable forested ecosys-tem This island faces the Asian continent across theEast China Sea, and rainfall and some tree (Pinusamamiana) on the island are intensely affected byaerosols from the surrounding sea and by acidic mate-rials, including gases (SOx and NOx) and aerosols,transported from China (Satake et al.1998; Nakano et

al 2000; Nagafuchi et al 2001; Kume et al 2010).The annual average pH of rain on Yakushima is 4.7, avalue equivalent to that on the main islands of Japan(Tamaki et al 1991; Japan Environmental SanitationCenter2002) However, the mean annual precipitation

on Yakushima ranges from 2,500 to 4,700 mm at loweraltitudes along the coast, and it exceeds 8,600 mm inmountainous areas (Eguchi 1984) These amounts arethree to five times the precipitation on the mainJapanese islands, indicating that Yakushima is receivingproportionally higher total inputs of acidic materials inprecipitation Further, the geology of Yakushima iswidely composed of granite, which is known to havesmall acid neutralization capacity Nevertheless, theimpact of rain and dust minerals from the Asian conti-nent on the island’s plants, soils, and streamwater isunclear This study was undertaken to elucidate thegeochemical and Sr and Nd isotopic characteristics

of Yakushima’s aquatic, soil, and vegetation systemsand their responses to these atmospheric inputs

2 Study Site and Methodology2.1 Geography and Geology

Yakushima Island is located 70 km south of Kyushu(30° N, 130° E), Japan’s third largest island Thissmall island, 132 km in circumference and 503 km2

in area, consists of steep mountains covered by densenatural forests with many cliffs and with many water-falls owing to the large amounts of precipitation Mt

Pacific Ocean Sea of Japan

Yakushima Granite

8182

50 5136

7543

2824

22

20 38

2725

21

343231

2326

2930

33

3576

5253

59606162

74

64656669

40

44

7273

80

78 77

798483

Taino R.

Sampling site of rain

Onoaida

Shiratani-UnsuikyoShin-Takatsuka Mt Tachudake

AmbouIssou

Fig 1 Upper map location

of Yakushima Island and the

study sites: I, Yakushima

Is-land; II, Tanegashima; III,

northern Kagoshima; IV,

Naegi; and V, Tsukuba

Bot-tom map sampling sites

locations and geological

background of Yakushima.

Red circles, black triangles,

and empty squares represent

streamwater sampling points

in areas with bedrock

domi-nated by granite, bedrock

dominated by sedimentary

rocks of the Kumage group,

and mixtures of the two

types of rocks, respectively.

The large empty square

indicates the Yotsuse sample

site discussed in the text.

Three large filled squares

indicate the locations of the

rainwater monitoring and

sampling by Tamaki et al.

Sampling locations in areas

with granitic bedrock are 23,

39, 45, 57, 66, and 83, and

those in areas with Kumage

sedimentary rocks are 14,

60, and 82 The sites where

both streamwater and

bed-rock were collected are

shown in boldface

Dia-monds indicate the sampling

sites of soil in Nakano et al.

Miyanoura, the highest point on the island, at 1,935 m

above sea level (a.s.l.), is also the highest peak in the

Kyushu region The annual mean temperature is

around 20 °C at the coast; this corresponds to the

margin between the subtropical and warm temperate

zones (Tagawa 1994); however, the temperature

decreases with increasing elevation, and areas above

1000 m a.s.l receive snow in winter Accordingly,

there are distinct altitudinal zones of vegetation

About 14,000 residents live in small areas of

Yakushima, mostly along the coast at elevations less

than 100 m a.s.l

The island is composed mainly of Miocene granites

of the ilmenite series that contain orthoclase

mega-crysts with maximum lengths of 14 cm, as well as

plagioclase, quartz, and biotite, with small amounts of

chlorite, apatite, zircon, tourmaline, muscovite, and

ilmenite (Sato and Nagashima 1979) Anma et al

(1998) classified Yakushima’s granite into four types

on the basis of its occurrence, texture, and

petrochem-istry: the Yakushima main granite, the core

granodio-rite, the core cordierite granodiogranodio-rite, and the core

cordierite granite The Yakushima main granite

occu-pies 90 % of the total area of the Yakushima pluton,

whereas the other granites are locally distributed The

Yakushima granite body is an intrusion within the

Kumage Group, which originated in the Paleogene

age and is composed mainly of sandstone and shale

distributed around the periphery of the island These

sedimentary materials are sometimes overlain

uncon-formably by terrace deposits, talus deposits, and

Quaternary alluvium, mainly along the eastern and

southern coasts A pyroclastic flow deposit called

Akahoya covers these rocks in some areas

2.2 Samples

There are three sites (Issou, Tachudake, and Ambou in

Fig 1) for monitoring the precipitation chemistry in

Yakushima Detailed compositional data are available

for the Issou site, where rainwater was collected with a

bulk sampler at intervals of 1 or 2 weeks from 1994 to

1996 This site is about 250 m above sea level and 5 m

from the ground, on top of a building, and trees, as

viewed from the sampler, cover less than 30° of the

sky (i.e., there is little or no interference from trees)

From 1996 to 1997 we sampled streamwater at 79

locations chosen on the basis of their basin geology

during the baseflow period from summer to autumn

(Fig 1) These samples were divided into threegroups: those in granite-dominated watersheds, those

in watersheds dominated by the Kumage sedimentaryrock, and those in watersheds that include both types

of rock For comparison of streamwater quality inrelation with the watershed geology, we sampledstreamwater from several areas with a range of geo-logical conditions and with negligible upstream hu-man activity on Tanegashima Island, which is close toYakushima and composed primarily of sedimentaryrock; in the northern part of Kagoshima Island, which

is composed primarily of granitic rock, sedimentaryrock, and volcanic rock (mostly andesitic); in theNaegi area of Chubu district, which is composedmostly of granite; and in the Tsukuba area of Ibarakiprefecture, which is composed mostly of granite andgabbro (Fig.1) At each site, the water samples werefiltered through disposable cellulose acetate filterswith a pore size of 0.2 μm; pH and alkalinity weremeasured immediately after sampling

We also collected eight granite samples at six tions and four samples of the Kumage sedimentaryrocks at four locations (Fig.1) Soil is well developed

loca-on the hills and gentle slopes At the Yotsuse site in thenorthwestern part of Yakushima, facing the Asiancontinent (Fig.1), we collected samples of three plantspecies and soil samples at seven depths This site islocated at the top of a small hill (200 m a.s.l.), wherethe granitic bedrock is deeply weathered to producehorizons in the soil profile; the thicknesses of the Ahorizon and the B horizon were 30 and 170 cm, re-spectively, whereas the C horizon reached a depth ofmore than 500 cm

2.3 Analysis

We dried about 40 g of soil from each horizon night at 105 °C in an oven The dried samples werethen reacted with 10 % v/v hydrogen peroxide (H2O2)solution in a tall beaker at 70 °C to separate theorganic fraction The solution was then centrifuged(Kokusan Enshinki, H-103N Series) at 2,400 rpm for

over-30 min The supernatant was used for the Sr isotopeanalysis The residual fraction was washed with ultra-pure water; after centrifugation for 30 min, this super-natant was then discarded We collected residual soilsafter repeating this rinse procedure three times Threesoil fractions (<2, 2 to 20, and >20μm) were separatedfrom about 10 g of the residual soil by means of

Stokes’ law gravity sedimentation in deionized water.

They were then concentrated by centrifugation Bulk

soils and these fractions were digested with a solution

of HF, HClO4, and HNO3 We also extracted soil

samples of about 0.5 g with 1 N acetic acid (HOAc)

solution to remove the exchangeable fraction The

remaining solution was used for the Sr isotope

analy-sis Rock samples were pulverized in a tungsten

car-bide vessel with a HERZOG HSM-F36 disk mill

(HERZOG Automation Corp., Osnabrück, Germany)

to obtain powdered samples for chemical and Sr

iso-tope analysis All reagents used in this leaching and

dissolution procedure were of analytical grade or

better

Chemical analyses were performed at the Chemical

Analysis Center and the Institute of Geoscience,

University of Tsukuba The concentrations of cations

and anions in streamwater were determined by means

of inductively coupled optical emission spectrometry

(Jarrell Ash ICAP-757V, Kyoto, Japan) and a

Yokokawa Analytical Systems (Yokogawa, Japan)

IC7000 ion chromatograph, respectively The

chemi-cal compositions of the rocks and soils were

deter-mined by means of X-ray fluorescence with a Phillips

PW1404 analyzer We determined Sr and Nd isotope

ratios by using a Finnigan MAT 262RPQ mass

spec-trometer at the University of Tsukuba and a Thermo

Fisher TRITON mass spectrometer at the Research

Institute for Humanity and Nature The mean

87

Sr/86Sr ratio of nine standard NBS987 samples

dur-ing this study was 0.710246 (2σmean, ±0.000022; n09)

using the MAT262 RPQ and 0.710278 (2σmean,

±0.000012; n05) using the TRITON, and all

measure-ments were normalized with respect to the

recommen-ded87Sr/86Sr ratio of 0.710250 The143Nd/144Nd ratio

of the La Jolla standard was 0.511846 ± 0.000011

Streamwater was classified into three types based on

the geology of the upstream watershed of the sampling

point The chemical compositions of dissolved ions in

the streamwater of Yakushima (Table1) showed a largegeographical variation, but did not differ significantlybetween the samples from watersheds with graniticbedrock and those with Kumage Group bedrock Themean water quality values for streamwater inYakushima for the two type’s watershed geology andthose from the other study areas are summarized inTable2 Streamwater from all areas except Yakushimawas neutral to slightly alkaline, but there was a tendencyfor the streamwater in granitic watersheds to be slightlymore acidic than those in watersheds with sedimentary

or volcanic rock (Fig.2); the average pH (±σmean) valuesfor streamwater in the granitic watershed (III, IV, and V

in Table 2) and in watersheds with sedimentary orvolcanic rock (II, III, and V in Table 2) were 6.87±0.28 and 7.26±0.27, respectively

This difference is consistent with the composition ofgranite, which is composed mainly of minerals that areresistant to chemical weathering (i.e., quartz and potas-sium feldspar) and that thus have a lower capacity tobuffer acids in the rain One remarkable feature is thatthe Yakushima streamwater was more acidic than that inthe other basins, with pH ranging from 5.4 to 7.1 (anaverage of 6.5) versus a range of 6.7 to 8.0 at the othersites Furthermore, the streamwater at the other sites wasgenerally a CaHCO3 or NaHCO3 type, whereas theYakushima streamwater was generally a NaCl type.The average Na and Cl concentrations in theYakushima streamwater were about 7.6 and 4.7 timesthe average Ca and HCO3concentrations, respectively.Monthly analysis of the rainwater composition at theIssou site (Satake et al 1998; Nakano et al 2000)revealed that the concentrations of the major dissolvedions were high in winter and low in summer, but that theproportions of Na, Mg, and Cl (Fig.2in Nakano et al

2000) were roughly constant throughout the year andwere almost identical to those in seawater, indicatingthat these three ions are largely of sea salt origin Thenon-sea salt (NSS) Ca and K fractions in the Yakushimarainwater were 0.6 ± 0.2 and 0.3 ± 0.2, respectively(Fig 4 in Nakano et al 2000) Nakano et al (2000)suggested from their Sr isotope study that the NSS Ca isderived mainly from plant cover on Yakushima thatdominantly contains Sr with a marine isotopic signature.Table2 provides the mean pH, electrical conductivity,and concentrations of the main ions in precipitation.Chloride is assumed to be a conservative tracer forthe input of sea salt aerosols (Berner and Berner1987),and the ratio of a given cation to the Cl concentration in

streamwater therefore increases as a result of addition of

the cation to soil water through chemical weathering

The concentrations of the major cations (Na, K, Ca, and

Mg) in Yakushima streamwater were positively

corre-lated with the Cl concentration (Fig.3) In addition, the

Na/Cl and Mg/Cl ratios of the Yakushima streamwater

were close to those of seawater (Table2) Although the

Ca/Cl and K/Cl ratios of the Yakushima streamwater

were considerably higher than those of seawater

(Table 2), the values were still closer to the seawater

ratios than to those of streamwater from other areas of

Japan These results strongly suggest that the acidic

precipitation on Yakushima contains a substantial sea

salt component, which in turn controls the chemical

composition of dissolved elements in the Yakushima

streamwater

The sea salt component of the rain generally

decreases with increasing distance from the coast

(Berner and Berner1987) Tamaki et al (1991) reported

the average elemental composition of wet precipitation

over 2 years at two Yakushima sites with different

altitudes, Ambou at 40 m a.s.l and Tachudake at

475 m a.s.l (Fig 1) They found that rainfall onYakushima had a lower annual average Cl concentration

at 475 m than at 40 m, but had the same annual average

pH value (4.7) The concentration of Cl in the water of Yakushima, where the watershed is small,tended to decrease with elevation (Fig 4) At severallow-elevation sites, the Cl content of the streamwaterwas very high (>0.5 molL−1) Because of the highhumidity that results from the heavy rainfall in the studyarea, this high Cl content in streamwater cannot beexplained only by the concentration process that resultsfrom the evaporation of rainwater; instead, it suggeststhe dry deposition of sea spray in areas near the shore.The positive correlations of cations in Yakushimastreamwater with the Cl concentration indicate that thecation concentrations tend to decrease with elevation.The altitudinal decreases of Cl and cation concentrations

stream-in streamwater are likely to be caused by the stream-increasedcontribution of rainfall at higher elevations, whichincreases the amount of water relative to the amount ofsea salt The correlation coefficient between pH andelevation for the Yakushima streamwater is −0.42(P<0.05), suggesting that, although the correlation isweak, streamwater at high elevations tends to be moreacidic (Fig.4) A similar pattern of decreasing stream-water pH with elevation has been observed in base-poorwatersheds at the Hubbard Brook Experimental Forest(New Hampshire, USA) that have been affected byinputs of acidic deposition (Palmer et al 2005).Accordingly, the altitudinal decrease of Yakushimastreamwater pH value may be partly ascribed to theeffects of the larger amount of acidic rain because themountainous area receives a high input of H+ions fromthe atmosphere, and these cannot be fully neutralized byweathering of the granitic bedrock

NSS sulfate (NSS SO4) is an important componentresponsible for the formation of acid rain Nakano et al.(2001a) and Ebise and Nagafuchi (2002) reported thatstreamwater in the northwestern side of Yakushima is-land contained higher concentration of NSS SO4 thanthat in the southeastern side (Fig.5), and attributed thisareal variation of streamwater to acidic deposition trans-ported from the northwestern Asian continent However,the concentration of non-sea salt cations such as NSS Ca

in streamwaters of the granite watershed did not show ameaningful difference between the northwestern andsoutheastern sides compared to their altitudinaldecreases This result suggests that the chemical weath-ering of granite is affected by the amount of precipitation

Fig 2 Frequency distribution of streamwater pH in (upper

graph) watersheds with granitic bedrock and (lower graph)

watersheds with bedrock from the Kumage series of clastic

sedimentary rocks, with volcanic rocks (mostly andesitic), and

with gabbro Vertical dashed lines represent neutral pH (7.0)

and other acids such as carbonic and/or organic acids

generating in soil–vegetation system rather than the

atmosphere-derived NSS SO4

3.1.2 Rainwater Contribution to Cations

in Streamwater

When there is no direct contribution from human

activity, the ions dissolved in the streamwater should

ultimately originate from the atmosphere and from the

watershed’s bedrock In other words, dissolved ions in

streamwater as well as in the soil water can be rated into an atmosphere-derived component and abedrock-derived component (BDC), which can besubclassified into granite BDC for the BDC from thegranites and Kumage BDC for the BDC from theKumage sedimentary rocks It is generally assumedthat Cl in streamwater is derived mainly from precip-itation when there is no volcanic gas or evaporites(both of which are enriched in Cl) in the watershed(Berner and Berner1987; Nakano et al.2001b; Négrel

sepa-et al 2005) On the other hand, bedrock is generally

0.2

0.4 0.5

0.3

0.1

0.2

0.4 0.5

0.3

0.1

0.7 0.6

Fig 3 Concentrations of the four major cations (Na, Mg, K,

Ca) as functions of the Cl concentration in streamwater on

Yakushima and in other areas with different bedrock geologies.

Solid lines indicate values based on the ratios in seawater

mean compositions in rainwater at two sites [elevations of

475 m (high Cl) and 40 m (low Cl)] on Yakushima in a study